If Michigan State basketball wasn't plagued by injuries, perhaps Matt Costello could cut himself more slack.

But faced with the reality that without Adreian Payne the No. 4 Spartans (16-1, 5-0) desperately need his presence in the post, he's frustrated by not being able to do more -- be better now.

"It's never really been a set thing (how much I play)," Costello said Thursday after practice. "It's been coaches just trusting me, seeing how I feel.

"(Against Northwestern), I was a little tired, played like crap, everybody saw it. I'm still going to struggle with it every once in a while, especially with conditioning and getting back into it."

Matt Costello played his best game of the season against Minnesota, but took a step back this past Wednesday versus Northwestern. AP Photo

Costello was sidelined for four games and countless practices for the better part of December with mononucleosis.

Even though the sophomore forward returned in time for the Spartans' Big Ten opener at Penn State on Dec. 31, head coach Tom Izzo said over and over that they would ease Costello back in to basketball. Citing his own daughter's experience with mono, Izzo's concern of a relapse weighed on the coach heavily.

Now, six weeks after his original diagnosis, Costello is getting close. Izzo says there are no more restrictions on the sophomore. Costello played a season-high 26 minutes versus Minnesota this past Saturday, one of the best games of his career, and finished with seven points and eight rebounds.

But Wednesday in Michigan State's 54-40 win over the Wildcats, he took a step back. He played just 15 minutes, was visibly exhausted early in the first half and finished with just four points and one rebound.

While Izzo was understanding of Costello's fatigue, the team can no longer afford to have its starting big man produce just a single rebound. And Costello knows that.

"It was real frustrating," said Costello, who's averaging 3.1 points and 3.2 rebounds. "I mean, I'm a body, I can play. But I can't play where I know I can. It is frustrating. But, you got to give it what you got and you got to use what you got. I'm going to give as much as I can. Al (Gauna) is going to give it as much as he can, Gavin (Schilling) is going to give it as much as he can while (Adreian Payne) is out and he'll be ready when he comes back.

"I got to break (the mindset of fatigue) and try to roll all the time."